1. Field of the Invention
The present invention relates to an exposure apparatus and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate very fine semiconductor devices such as a semiconductor memory and logic circuit using photolithography (exposure). The projection exposure apparatus projects and transfers the circuit pattern formed on a reticle (mask) onto, e.g., a wafer via a projection optical system. Along with the recent micropatterning of semiconductor devices, there are proposed various kinds of resolution enhanced technologies for resolving a pattern having a line width equal to or shorter than the wavelength of an exposure light source. One of these resolution enhanced technologies is a so-called modified illumination method (oblique illumination method). The modified illumination method does not illuminate a reticle with illumination light having a uniform angular distribution, but obliquely irradiates a reticle with illumination light. Examples of the modified illumination method are an annular illumination method, dipole illumination method, and quadrupole illumination method. In the modified illumination method, the angular distribution of a light beam which illuminates a reticle corresponds to the positional distribution of the pupil plane (Fourier transform plane) relative to a reticle (object plane).
The light intensity distribution of a light beam from a light source on the pupil plane is an axisymmetrical Gaussian distribution. To shape the light intensity distribution on the pupil plane into an annular shape, dipolar shape, or quadrupolar shape, an optical system for transforming the light intensity distribution is necessary. Examples of the simplest optical system which transforms the light intensity distribution are dipolar and quadrupolar aperture stops arranged on the exit surface of an optical integrator corresponding to the pupil plane. However, the use of an aperture stop extracts a component of the light beam from the light source. This makes it impossible to efficiently utilize the light beam from the light source, resulting in a decrease in illuminance on the reticle.
Japanese Patent Laid-Open No. 7-201697 proposes a technique of transforming the light intensity distribution on the incident surface of the optical integrator using a diffraction optical element. This technique can obtain a desired light intensity distribution without using any aperture stop.
To optimize modified illumination for the reticle, it is also proposed to change the intensity of an annular light beam (zone ratio) or the magnitude of the coherency σ (i.e., the numerical aperture of the illumination optical system on the exit side/the numerical aperture of the projection optical system on the incident side). For example, the zone ratio can be changed using an optical element having a conical concave incident surface and a flat exit surface, and an optical element having a flat incident surface and a conical convex exit surface. When these two optical elements are inserted in the optical axis, they can change the zone ratio or coherency a by forming an annular light beam and changing their interval. The coherency σ can be changed using a variable magnification zoom optical system.
Unfortunately, only changing the zone ratio or coherency σ is insufficient to optimize modified illumination (i.e., to optimize light which illuminates a circuit pattern to be transferred). For example, even when the zone ratio or coherency σ is changed, adjacent patterns may merge or an isolated pattern may become thin due to the influence of the proximity effect.